For environmental protection, improved fuel efficiency, lower emission and higher power are increasingly needed to internal engines, and high-load combustion and high-load engine specification require combustion chamber parts to have higher wear resistance in a wide use temperature range. Valve seats used with intake valves and exhaust valves for keeping the gas tightness of combustion chambers are exposed to combustion pressure, and repeated shock by the motion of valves, needing wear resistance in a special environment. Particularly in fuel direct injection engines in which fuel is directly injected into each cylinder (cylinder bore), there is a hard lubrication condition in contact portions of valves and valve seats, because a fuel does not pass through them, and they are in a high-temperature environment because they are little cooled by the evaporation of a fuel. For valve seats for fuel direct injection engines, namely valve seats used under a hard lubrication condition at high temperatures, for example, JP 2003-166025 A discloses an iron-based, sintered alloy in which solid lubricants are dispersed to improve self-lubrication, and a high-alloy material having improved wear resistance at high temperatures.
However, the addition of solid lubricants in a predetermined amount or more reduces the strength of a sintered body, resulting in insufficient wear resistance at low temperatures.
Valve seats are required to have high finish precision in surfaces brought into contact with valves to secure gas tightness in combustion chambers, and excellent machinability for coaxial machining with valve guides after assembled to cylinders. However, valve seats are harder to machine than other parts constituting engines, because of high-hardness particles, etc. added to improve wear resistance, and so-called intermittent cutting due to voids in the sintered alloy, thereby reducing productivity in an engine-producing line. Thus, valve seats are required to have improved wear resistance and machinability.